The present invention generally relates to measurement devices. More specifically, the invention relates to interferometers.
In general, optical interferometry is the act of splitting and recombining electromagnetic waves, in particular, visible light waves, to measure surface geometries, distance, etc. The advancement in interferometry has come in many avenues of technology. Long-range telescopes, high-precision spectrometers, compact disc players, etc., use some form of interferometry. Micro-machinery is a growing technology field that often utilizes interferometers because, they typically have high resolution and precision. In general, displacement measurements in the sub-nanometer range can be detected with today""s interferometers. To examine microscale structures, the lateral resolution of the interferometers, generally, need to be improved. This can be achieved by coupling the interferometer to a regular microscope. Unfortunately, the size of the interferometer becomes rather large and subsequently may not fit in small spaces for inspection. Furthermore, to inspect a large number of microscale structures either the sample or microscope objective is scanned, resulting in slow imaging.
In order to obtain interferometric measurement sensitivity in a small volume, several methods have been developed. One of these methods involves phase sensitive diffraction gratings as described in a technical paper entitled xe2x80x9cInterdigital cantilevers for atomic force microscopy,xe2x80x9d published in Appl. Phys. Lett., 69, pp. 3944-6, Dec. 16, 1996 by S. R. Manalis, S. C. Minne, A. Atalar, and C. F Quate and also in U.S. Pat. No. 5,908,981 to Atalar et al.
Similar structures are also used in microaccelerometers to measure the displacement of a control mass with interferometric precision as described in a paper written by E. B. Cooper, E. R. Post, and S. Griffith and entitled xe2x80x9cHigh-resolution micromachined interferometric accelerometer,xe2x80x9d Appl. Phys. Lett., 76 (22), pp. 3316-3318, May 29, 2000. It should be noted, however, that these papers discuss measuring relative distance of the object with respect to the reference gratings.
Two well known uses for microinterferometers are range finding and shape measurement, of which there are several optical range finding and shape measurement methods. Traditional range finding using focus analysis is an effective method, but for high accuracy and reduced depth of field, the lenses are typically large. Hence, mechanical scanning to make shape measurement becomes a slow and difficult task. Microscopes can be used to enhance the resolution, but this comes at the cost of extremely short standoff distances from the object, making scanning difficult. Interferometric ranging methods are very accurate, but in ordinary implementations, the methods operate in a relative coordinate space and can be problematic when the object surfaces have abrupt discontinuities.
It would be desirable to have a microinterferometer that can determine an absolute distance as well as relative distance, as opposed to most of today""s microinterferometers which can determine only relative distance. It would also be desirable to increase the resolution and sensitivity of the microinterferometer, while keeping the microinterferometer relatively fast enabling measurement of the dynamic response of the microstructures under investigation.
At some point, the sensitivity, and thus the resolution of the microinterferometer can be improved only so much. Like most transmission/receiving systems, this occurs when miniscule differences in the signal can not be detected because of the noise in the system. Once the signal strength, in this case the intensity of the light and the lateral resolution, has been improved to its known limit, generally, the next step is to reduce the noise floor. This, similar to increasing the strength of the signal, increases the signal-to-noise ratio (SNR). In optical systems such as interferometers several noise sources exist. For example, noise caused by the emitting light source, shot noise in the receiving element, electrical noise from backend electrical components, and overall system noise, such as mechanical and thermal noise. It would be desirable to have a microinterferometer that can overcome and/or reduce some or all of the noise in the system. As mentioned, this can increase the SNR and thus improve the overall resolution and performance of the microinterferometer.
Based on the foregoing, it should be appreciated that there is a need for improved microinterferometers that address the aforementioned problems and/or other shortcomings of the prior art.
The present invention relates to microinterferometers for accurately measuring the distance to an object surface. In this regard, one embodiment of a microinterferometer includes a substrate and a tunable, phase-sensitive, reflective diffraction grating formed atop said substrate. The diffraction grating is configured to reflect a first portion of an incident light and transmit a second portion of the incident light, such that the second portion of the incident light is diffracted. The diffraction grating being further configured to be controllably adjusted. The microinterferometer also includes a photo-detector for receiving interference patterns produced from the first portion of the incident light reflected from the diffraction grating and the second portion of the incident light reflected from the object surface. The microinterferometer also includes a controller coupled to the photo-detector and the diffraction grating for adjusting the diffraction grating, such that the interference patterns are altered.
Methods for optimizing the performance of a microinterferometer are also provided. One such method, among others, is practiced by the following steps: enabling operation of the microinterferometer to measure the distance to a target surface; calculating the distance to the target surface from measurements made by the microinterferometer; and controllably adjusting a tunable diffraction grating of the microinterferometer to optimize the performance of the microinterferometer.
Systems for optimizing the measurements of a microinterferometer are also provided. One such system, among others, includes means for receiving data from the microinterferometer, means for processing the data from the microinterferometer; and means for generating a feedback signal, the feedback signal being related to the data processed by the means for processing and configured to tune a tunable diffraction grating of the microinterferometer.
Methods of fabricating a tunable diffraction grating are also provided. One such method, among others, is practiced by the following steps: providing a substrate; forming an electrode on the substrate; and forming a phase-sensitive, reflective, tunable diffraction grating above the electrode and atop the substrate, such that the diffraction grating is positioned a variable distance away from the electrode.
Other systems, methods, features, and advantages of the present invention will be or become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional systems, methods, features, and advantages be included within this description, be within the scope of the present invention, and be protected by the accompanying claims.